Dewar flasks are known apparati having a sealable or sealed, hollow housing, which is evacuated or substantially evacuated through application of as much vacuum as possible. The object of a Dewar flask is to insulate to the fullest extent. Thus, the vacuum is applied rather indiscriminately but so as to provide the lowest pressure inside the housing for the least amount of heat transmission. The Dewar flask is a known, effective insulator to keep substances hot or cold for extended periods, especially for cool materials, and it may be employed as a bath tub in viscosity testing.
In the art of testing oil and related fluid viscosity, one longstanding, well known test protocol is ASTM D 2983, which was developed in the 1950s by Theodore W. Selby. See e.g., SAE Transactions 68:457-67, 1960. In the ASTM D 2983 protocol, q.v., a lubricant fluid sample is cooled in an air bath at test temperature for 16 hours. The sample is then carried in an insulated container of balsa wood to a nearby sensitive rotational viscometer such as, for example, a Model LVT or LVTD viscometer available from Brookfield Engineering Laboratories, Inc., Stoughton, Mass., or a Scanning Brookfield PlusTwo (TM) viscometer available from Tannas Co., Midland, Mich., where Brookfield viscosity is measured at a predetermined temperature in the range from minus 5 to minus 40 degrees C. See, FIG. 1. This protocol has a number of disadvantages, to include as follows:
1) Repeatability of cooling of a given sample is not optimum due to the air cooling of samples, etc. Therefore, duplicate or triplicate samples are run to obtain an average value of viscosity in order to reduce inconsistency to some extent. PA1 2) Having to remove the cooled stator tube from the air bath and inserting it into the balsa block for the viscosity measurement interjects an undesired temperature increasing effect on the sample. Since viscosity generally decreases due to the rise in temperature, and a change in a few tenths of a degree in temperature can have a dramatic effect on viscosity, the test can be unreliable. PA1 3) A general plot of the response of a viscometer to a to a fluid versus time would show that initially values shown by the instrument quickly increase to the value reflecting the viscosity because of instrumental factors such as the typical rotational viscometer spring coiling. Next, the values decrease due to non-Newtonian properties of the sample such as shear degradation and gelation plus an increase in temperature. A general plot of viscosity versus temperature would show that the viscosity decreases in a sample as the temperature increases. The magnitude of these competing effects cannot be effectively distinguished thereby. PA1 4) A long time is required to run each test. Not only does sample cooling take the many hours, but time is required to transfer the sample, and to measure its viscosity. For example, 30 seconds is required at 30 or 60 rotations per minute (rpm) for lower viscosity liquids, but 5 minutes is required at 0.6 rpm for higher viscosity liquids with the protocol. The more viscous a liquid is, the more time the test runs take, and these highly viscous liquids are generally of critical import, but their viscosity measurement is of the poorest reliability with such long times required to run the tests, as undesired heating of the sample, etc., occurs. PA1 5) Large numbers of samples in duplicate are required to obtain the viscosity values over a meaningful range of temperature. Another protocol in the viscosity testing art akin to the ASTM D 2983 is the CEC L18-A-80, a European standard. In this European protocol, q.v, a liquid bath is substituted for the ASTM D 2983 air bath. See, FIG. 1. This protocol is not without its disadvantages, some of which follow: PA1 1) Gelation properties are different. PA1 2) Cooled samples are maintained in the liquid. PA1 3) The same results as ASTM D 2983 are not obtained.